1. Field of the Invention
The present invention relates to an anodic oxide and a process of anodic oxidation. In particular, the present invention relates to a process for anodically oxidizing electrodes and interconnections of semiconductor devices.
2. Prior Art
To prevent current leakage and short circuit from occurring in a semiconductor device, in general, a high-resistance anodic oxide is formed as an insulator by anodically oxidizing the surface of the metallic constituents such as electrodes, interconnections, etc., of the semiconductor device.
The characteristics of an anodic oxide depend on the electrolytic solution that is used in the anodic oxidation. In particular, in the case that a neutral electrolytic solution is used as the solution for the anodic oxidation, a dense and pinhole-free composition known as barrier-type oxide can be obtained. This anodic oxide is therefore advantageous, and is frequently used as an insulator to separate electrodes and interconnections.
In a prior art process of anodic oxidation, an anode of a direct current (DC) power source is connected to a metallic (e.g., aluminum, tantalum, titanium, or silicon) interconnection formed on a substrate while connecting a cathode made of platinum and the like to the cathode of the DC power source, and applying a DC voltage between the metallic interconnections and the cathode while immersing them into the anodically oxidizing solution. In general, the current and the voltage applied between the anode and the cathode are varied in the following manner.
Referring to FIG. 1, conventionally, the current is first maintained at a constant value for a certain period of time (constant-current mode). In this mode, the resistance of the metallic interconnection increases as the anodic oxide film formed on the metallic interconnection becomes thicker, thereby gradually increasing the voltage.
Upon reaching a predetermined value of voltage, the method for controlling the process is switched to realize a constant voltage mode in which the current flow is decreased by maintaining the voltage at a constant value. The voltage is maintained constant for a duration of several to several tens of minutes to complete the process of anodic oxidation.
In the aforementioned process of anodic oxidation, the film thickness of the anodic oxide can be controlled by changing the voltage. That is, because the anodic oxide has high resistivity, the voltage in the constant-current state increases proportional to the film thickness of the anodic oxide being formed throughout the anodic oxidation process.
However, the oxide film thus obtained is not sufficiently uniform in terms of film thickness and quality. Then, in order to improve the quality of the film, the oxidation is further continued while maintaining the voltage at a constant value. The current flow then decreases as to finally attain a value equivalent to about one tenth of the current maintained through the constant-current state.
However, when the anodic oxide film formed in the above manner is used on a gate electrode of an insulated gate field effect transistor, there have been observed some problems as discussed below.